I tried the 15" last week and thought the sound was pretty good. I was about to go pick up the Roland 120xl bass cube that I mentioned in a few posts up (it was only $279 and sounded great) but it sold yesterday. I haven't played the 12" Ampeg but I don't think I'd use all 200w of the 15" version, so if the 12" sounded nicer at low volumes then I'd go ahead and order one.
Keep in mind that the RB-115 only produces 200 W if an external speaker is used. It produces 100 W into the internal 15" speaker ... same as the RB-112.